Multimode fiber optic gyroscope

ABSTRACT

The invention concerns an optical fiber gyrometer with high stability of scaling factor comprising a light source (2) supplying the two ends of a ring-shaped guide (1) and a sensor (6) receiving the two modal distributions which have traveled along the guide (1) in mutually inverse directions, the values Vdc, V1, and V2 representing the continuous components respectively incoherent, of the fundamental and of the order 1 harmonic, of the optical signal are used for establishing a quotient independent of the fluctuations of the components of the gyrometer and representing the scaling factor. These values are derived for the first detection and for the two others from the synchronous demodulation of the optical signal. The invention is useful for users and manufacturers of gyrometers.

The invention concerns a multimode optical fiber gyroscope, single ormulti-axial, with improved resolution and stability.

The improvement essentially relates to diagonal stability and thescaling factor.

Ring-shaped gyroscopes with multimode optical fibers are known in theart. However, the prior art solutions do not present practical solutionsfor industry, nor do they achieve the objectives of the presentinvention or respond to industrial needs.

These prior art solutions depend upon the SAGNAC effect and often uponthe physical principle of reciprocity, according to which an extrinsicdisturbance produces the same phase effects on both counter-rotatingwaves propagating within a ring guide. These considerations make itnecessary for the system's inlet/outlet device to be equipped with areciprocal dual coupling, which increases manufacturing costs and has anegative effect on output.

Furthermore, prior art devices often resort to particular methods ofmodulation and detection which are difficult to implement and imposevery strict manufacturing limitations.

The essential advantage of the present invention consists in the use ofa source, an optical fiber, and a detection circuit in an economicaldevice, easily adapted to industry

To achieve these objectives, the components which are selected must meettwo essential criteria.

The first requirement is to obtain maximum dispersion between twoadjacent modes, no matter what modes are being considered. Thiscondition is fulfilled by making the appropriate selection first, of thesource characteristics, that is, its spectral width and central wavelength, and second, the parameters influencing the fiber's dispersioncharacteristics such as length, numerical aperture, index profile, andother opto-geometric characteristics of the fiber.

The preferred version of the invention has a source with a broadspectrum associated with a highly dispersive fiber.

The second criterion concerns the symmetry of modal distributions inboth propagation directions.

The principal goal of the present invention is to furnish an outputvalue proportionate to the speed known as rotation speed, that is,angular displacement, which is independent of the principal diversefluctuations of the gyroscope components.

According to one variation, the invention furnishes data about theindependent rotation speed of the various principal fluctuations in thegyroscope components. Until now, variations in the source wave lengthhave remained difficult to quantify.

From that point, in a broad range of operating temperatures, it is nolonger necessary to take multiple precautions to minimize variations inthe source wave length.

Other advantages of the invention are enumerated below:

The use of multimode fiber with a large core diameter and largenumerical aperture allows the use of a strongly divergent light sourcewithout affecting energy output at injection. A simple, inexpensive,light-emitting diode accomplishes this. The same holds true for otherconnection operations.

It is no longer necessary for the design of the gyroscope to incorporatetwo reciprocal input/output couplers. A single X coupler can be used,which decreases manufacturing costs and improves the power output of thesensor.

Since the light signal within the fiber is completely depolarized, it isno longer necessary, as with monomode fiber gyroscopes, to usepolarizers, polarization controls, or depolarizing systems. In addition,the sensor becomes completely immune to magnetic fields (Faraday effect)and electrical fields (Kerr effect). It is no longer necessary to useexpensive materials to block these effects. This reduces the number ofcomponents substantially and simplifies assembly.

The system's stability depends upon the optical properties of itscomponents. This results from the possibility of assimilating thegyroscope with a large number of independent, elementary gyroscopes.Thus, it is no longer necessary to reset the gyroscope to zero beforeeach use, and systems can be stored and put to immediate use asnecessary.

The ability to manufacture a small size gyroscope is an importantadvantage in certain applications requiring mini-gyroscopes.

The signals chosen, that is, 0^(th), 1^(st) and 2^(nd) order harmonicsin the mathematical formulas adopted, result in gyroscopes with ascaling factor which is independent of the principal fluctuationscharacterizing gyroscopes of this type.

Other features and advantages of the present invention will be apparentfrom the following description, given by way of example, of onepreferred embodiment, with reference to the accompanying drawings, inwhich:

FIG. 1 represents the principle of the physical effect of modepropagation and coupling;

FIG. 2 is a schematic representation with functional blocks of agyroscope according to the invention in the case of a first formula forprocessing the output signal;

FIG. 3 is a schematic representation in functional blocks of a gyroscopeaccording to the invention in the case of a second formula forprocessing the output signal;

FIG. 4 is a partial schematic representation of a variation of thegyroscope with a single Y coupler;

FIG. 5 is a partial schematic representation of the gyroscope of FIG. 1with amplitude modulation of the light source;

FIG. 6 is a schematic representation of the principle of a multi-axialgyroscope;

FIG. 7 is a schematic representation of a simplified version of amulti-axial gyroscope;

FIG. 8 is a schematic representation of a gyroscope with three axes andthree oscillators;

FIG. 9 is a schematic representation of a variation of the multi-axialgyroscope with switching phase modulators.

The primary goal of the invention is to eliminate the principal causesof instability, using units and circuits giving output values which,when mathematically associated, provide a representation of rotationspeed, that is, the gyroscope's angular displacement. This value isindependent of the variations originating from various fluctuationsassociated with the components of the gyroscope. According to onevariation of the invention, the result is also independent of variationsin the source wave length.

Furthermore, to ensure that the gyroscope functions adequately,appropriate selections must be made first, among source characteristics,that is, spectral width and central wave length, and also, among theparameters influencing fiber dispersion characteristics such as length,numerical aperture, index profile, etc.

In actuality, any intrinsic or extrinsic disturbance can cause twonearby co-propagating modes of any order to couple and produceinterference, causing an unstable gyroscope response. To overcome thisproblem, the two modes must be totally independent in order to renderany coupling of the modes as incoherent as possible.

This condition can be verified by selecting an intermodal dispersionfiber such that the difference in optical path is greatly superior tothe coherence length of the light source. Since a fiber's intermodaldispersion depends upon its index profile, a fiber of high index with anindex profile coefficient tending towards infinity easily fulfills thiscriterion when associated with a wide spectrum source.

Besides eliminating the influence of fluctuations among the componentson the outcome, the stability of the gyroscope according to theinvention results from the physical effect of modal averaging and uponthe principle to be described in conjunction with FIG. 1, which showsone section of optical fiber of arbitrary length L modeled like a systemwith M groups of independent modes with double inlets/outlets on whichthere are defined two opposite directions of mode propagation denotedas + and −, respectively.

Among the many independent modes that can be propagated in the fiber,let us consider any two nearby modes k and l, denoted as (k⁺, l⁺) and(k⁻, l⁻), respectively, for each direction of mode propagation withinthe fiber.

Because there is independence between modes, combinations of modes maybe either coherent or incoherent. For example, modes may result fromcoupling between two direct modes (k⁺, k⁻) and coupled modes (lk⁺, lk⁻)resulting from nearby mode 1 which is coupled in mode k. The differentpossible combinations at the level of mode k are detailed as follows:

combination between direct modes:

the combination of modes (k⁺, k⁻) gives a reciprocal interferencesignal, that is, one which contains no term of disturbance;

combination between direct modes and coupled modes:

the combinations (k⁺, lk⁺) and (k⁻, lk⁻) correspond to combinationsbetween co-propagating and incoherent modes, as they originate from twoinitially independent, superimposed modes k and l. Thus, thesecombinations do not produce any interference and they contribute only tothe incoherent optical signal.

the combinations (k⁺, lk⁻) and (k⁻, lk⁺) correspond to combinationsbetween modes which are counter-propagating but incoherent, since theyoriginate from two independent modes. As described above, they produceno interference and they contribute only to the continuous opticalsignal.

combination between coupled modes:

the combination (lk⁺, lk⁻) corresponds to a combination betweencounter-propagating modes originating from the initially incoherentmodes l⁺ and l⁻. Nevertheless, modes (lk⁺, lk⁻) travel different opticalpaths in the fiber, which generally ensures their independence, exceptin one particular case, which will now be examined.

It is important to consider the case of modes originating from couplingsdue to stationary, localized, symmetrical disturbances along the coil,for example at Z⁺ and Z⁻, as shown schematically in FIG. 1. Thecombinations among modes created respectively at Z⁺ and Z⁻, i.e.(lk⁺(Z⁺),lk⁻(Z⁻)) and (lk⁻(Z⁺), lk⁺(Z⁻)) remain coherent to the extentthey travel essentially the same optical paths in the fiber. Thedisturbances are not necessarily identical and thus they producedephasing, which causes the appearance of non-reciprocal terms and inturn produces instability in gyroscope response.

This demonstration makes reference to a given mode of propagation k. Itwill now be extended to all the possible modes along the total length ofthe fiber.

The resulting output signal appears as the sum of two components:

a first incoherent, continuous component originating from couplingsbetween two incoherent modes; and

a second coherent component which consists of the sum of the signalsoriginating from the combinations between direct, similar modes withSagnac phase terms, and of signals originating from combinations betweencoupled modes with simultaneous Sagnac phase terms and instabilityterms.

It is desirable to decrease the influence of instability terms. Onesolution would consist of using a highly multimodal fiber. In practice,the higher the possible number of modes within a fiber, the higher thenumber of possible combinations between modes, and the greater thetendency of the average value of random instability signals to approachzero, thus rendering the gyroscope intrinsically stable. However, thissolution has one drawback. An increase in the number of modes within thefiber is accompanied by an increase in the incoherent, continuouscomponent of the signal and thus of the phototonic noise it generates.This results in degradation of the signal/noise ratio and thus in signalresolution. Calculations show that this is inversely proportionate tothe total number of possible modes in the fiber.

Thus, a compromise is sought between stability and signal resolution,that is, maximum intermodal dispersion with a simultaneous limit on thetotal number of modes in the fiber.

Calculations prove that all the terms originating from the coupled modescontribute to the global signal in the ratio of L_(cf)/L, where L_(cf)is the length of coherence in the fiber and L is its total length.

Thus, the total number of modes in the fiber can be limited withoutharming signal stability if, at the same time, the contribution ofinstability terms of said signal is limited by minimizing therelationship L_(cf)/L. This can be achieved either by increasing thetotal length L of the optical fiber or by decreasing the length ofcoherence L_(cf) in the fiber. The length of coherence L_(cf) in thefiber is proportionate to the difference in propagation times in themode groups of the order k and l denoted as Δτ_(l, k) with${\Delta\tau}_{l,k} = {\frac{{nL}_{cf}}{c}\Delta \quad {\left( \frac{2 - {2p} - a}{a + 2} \right)\quad\left\lbrack {\left( \frac{l}{m} \right)^{\frac{2a}{a + 2}} - \left( \frac{k}{m} \right)^{\frac{2a}{a + 2}}} \right\rbrack}}$

where:

n is the index of the fiber core,

c is the speed of light.

Δ is the relative difference of the core and the clad indexes,

p is the chromatic dispersion parameter of the fiber,

α is the profile of the fiber index, and

M is the total number of groups of modes.

The proportion of coherent energy generated by couplings between modes kand l is such that the difference in the optical path between these twogroups of modes is at least equal to the length of coherence of thesource:$\frac{L_{cf}}{L} = {\frac{L_{cs}}{L \cdot {ON}^{2}}\left( \frac{a + 2}{2 - {2p} - a} \right)\frac{1}{\left\lbrack {\left( \frac{1}{M} \right)^{\frac{2a}{a + 2}} - \left( \frac{k}{M} \right)^{\frac{2a}{a + 2}}} \right\rbrack}}$

where:

L_(cs) is the length of coherence of the source,

L is the total length of the fiber, and

ON is the numerical aperture of the fiber.

It follows from the preceding formula that the various parameters whichcan be manipulated to achieve the desired compromise between stabilityand resolution are as follows: index profile, central source wavelength, spectral width of source, chromatic dispersion parameter offiber, numerical aperture, fiber radius, core, and length.

The gyroscope shown schematically in its entirety in FIG. 2 comprises aring guide or coil 1 consisting of coiled optical fiber of variablelength depending upon the sensitivity sought, practically speaking,ranging from several meters to several hundreds of meters in length.According to the invention, the optical fiber must be a multimode fiberwith an index profile allowing a high degree of intermodal dispersion. Apreferred example of such a fiber is a multimode high index fiber.

As shown in FIGS. 2 and 3, a source 2 of slightly coherent light with abroad spectrum, preferably a light-emitting diode, supplies the twoinlets 3 and 4 of the coil 1 of optical fiber through a single X coupler5. This single coupler, which is also multimodal, symmetrically dividesthe modes in both directions, that is, it separates the wave emitted bylight source 2 into two symmetrical, counter-propagating modaldistributions.

X coupler 5 also collects waves which have passed through coil 1 inmutually inverse directions and directs them towards an interferencedetector 6, preferably a photodiode located on the output pathway of thegyroscope.

According to a variation of the execution of the invention shown in FIG.4, the ends of coil 1 are coupled to the unit consisting of the lightsource 2 and interference detector 6 by means of a single Y coupler 7.In this case source 2 and detector 6 are located at the same level onthe input branch 8 of coupler 7. Two operational modes are possible. Ina first operational mode, light source 2 and interference detector 6alternate periodically. In a second operational mode, light source 2 andinterference detector 6 work continuously and simultaneously.

The various optical elements of the gyroscope of the invention that arepreferably based on diffractive optical components have been examined.Next, the operating electronic circuits associated with them will beexamined.

Phase modulating elements such as optical signal modulator 9, knownsimply as modulator 9, are asymmetrically arranged at one of the inlets3 or 4 of the multimode optical fiber coil 1, in such a way that theentire length of optical fiber serves as a delay line, with theco-rotating wave immediately reaching modulator 9, while thecounter-rotating wave must first pass through the entire length of coil1 before reaching it.

There are other possible variations in execution for phase modulator 9,which are not shown.

A first, known variation uses a tube of piezoelectric materialsurrounded by a length of multimode optical fiber.

A second variation uses a portion of multimode fiber with the exteriorcoated with piezoelectric material. This solution is advantageous inlimiting the space required for phase modulating elements 9.

Another variation concerns a multimode fiber inserted in a flexiblepiezoelectric capillary.

Yet another variation concerns a fiber with a periodic modulation of itscore index along a portion of its length, which is obtained byphotoengraving or chemical treatment. The benefit of this variation isthat it allows phase modulation of the counter-rotating light wavesdirectly at the fiber core, eliminating the need for conventionalmodulation from the device. For this reason it offers significantadvantages in terms of cost and space.

The installation has a sinusoidal modulation oscillator 10 furnishingreference ω_(i) used to supply a suitable modulation signal ω_(i) tomodulator 9.

The detection circuit per se comprises, first, an amplifier 11 locatedafter interference detector 6 which drives a synchronous demodulator 12.Modulation signal ω_(i) delivered by sinusoidal modulation oscillator 10and exciting phase modulator 9 also serves as a reference signal for thesynchronous demodulator 12.

In order to neutralize fluctuations associated with phase modulator 9, aloop is used to control the phase and the amplitude of the modulatedsignal in relation to the modulation signal, which simultaneously servesas the reference signal for synchronous demodulator 12.

The characteristic fluctuations in modulator 9 can also be overcomethrough the use of a suitable compensation circuit (shown by dashedlines) acting on local oscillator 10 and the synchronous demodulatorthrough a modulator detector, located, for example, either inside ornext to modulator 9.

According to a preferred embodiment, two band pass filters 13 and 14 arerespectively provided, first, between amplifier 11 and synchronousdemodulator 12, and second, between demodulator 12 and sinusoidalmodulation oscillator 10. The purpose of these filters 13 and 14 isespecially to eliminate any non-linearity in frequency of phasemodulator 9. In practice, even when phase modulator 9 is excited with asingle frequency, it may begin to oscillate at modes corresponding tofrequencies other than the frequency of modulation. Therefore, it ispreferable to use band pass filters centered on the frequency ofmodulation signal ω_(i) from modulator 9 to eliminate such non-linearphenomena.

At the output of amplifier 11, there is a low pass filter 15 with lowfrequency cut-off to isolate the continuous incoherent component V_(dc)of the optical signal.

A much more precise solution than the simple low pass filtration ofcontinuous incoherent component V_(dc) of the optical signal is shown inFIG. 5. It consists of modulating light source 2 in amplitude by usingperiodic modulation voltage delivered by a generator V(f, t) at amodulation frequency f considerably lower than modulation frequencyω_(i) of the signal exciting phase modulator 9. A detection unit 16 inthe form of a dual rectangular synchronous demodulator then detects thecontinuous incoherent component V_(dc) of the optical signal. Thissolution offers the advantage of detecting very slight variations in thecontinuous incoherent component V_(dc) of the optic signal.

The assemblage of FIG. 2 terminates in a divider module 17 receiving avoltage V₁ originating from the synchronous demodulator, correspondingto a proportionality factor close to the amplitude of the component ofthe 1^(st) order harmonic (the fundamental) of the optical signal andvoltage V_(dc) corresponding to the continuous incoherent component ofthe optical signal. The role of the divider module is to effect thearithmetic relationship

S=V ₁ /V _(dc)

representing at a constant multiplication factor the scaling factor ofthe gyroscope, that is, the positive or negative algebraic value used todetermine rotation speed i.e., the speed of angular displacement of thegyroscope and thus of the support to which it is attached, which isdisplaced along with it and pivots about a reference axis in space. Thisrelationship in values is used to compensate for the majority ofvariations and fluctuations in intensity associated with instability ofthe optical source, losses in the fiber, the optical detector and othercomponents, resulting in the elimination of most of the fluctuationsconnected with fluctuations in the scaling factor of the gyroscope.

There remains, however, the fluctuation linked with the fluctuation inwavelength of the source wave, which tends to remain at a minimum whenusing the assembly and quotient method shown in FIG. 2.

Another assembly shown in FIG. 3 corresponds to another quotient. Again,the 2^(nd) order harmonic is used here. This quotient, indicated below,eliminates different fluctuations, but especially those caused fromwavelength variations in the source wave.

According to this variation, values V₁ and V₂ are detected and used inproportion to the amplitudes of the continuous coherent componentsoriginating from synchronous demodulation of the 1^(st) and 2^(nd) orderharmonics of the optic signal. These voltage values are found at theoutput of the synchronous demodulators associated with each 1^(st) and2^(nd) order harmonic.

The assembly shown in FIG. 3 is similar to that of FIG. 2. It comprisesthe same general functions.

There is, for example, an oscillator that can be programmed to generatethe excitation signal for modulator 9 at frequency ω_(i) through blockCAG which automatically controls the increase. The oscillator isequipped with two signal outputs at frequencies ω_(i) and 2ω_(i) towardthe two synchronous demodulators respectively set at frequencies ofω_(i) and 2 ω_(i), that is, at the modulation fundamental and its 2^(nd)order harmonic. Thus, at the output of the synchronous demodulators,signals V₁ and V₂ are obtained, corresponding to the amplitudes of thecontinuous coherent components of the fundamental and of the 2^(nd)order harmonic of the optical signal. The assembly is completed by aband pass filter whose role is to isolate V_(dc) which, as previouslyindicated, is the continuous incoherent component of the optical signal.These values enter a processor externally connected by a communicationsinterface. The interface is also connected to the processor and at oneinput, it receives a reference voltage Ref. V_(dc) for the value ofV_(dc).

This assembly also comprises an automatic control circuit to stabilizemodulator 9 or a compensation module similar to that of the assemblyshown in FIG. 2.

According to the invention, the processor establishes the followingmathematical relationship between values V₁, V₂ and V_(dc):$R = \frac{V_{1}}{\sqrt{{V_{2} \times V_{dc}}}}$

where

V₁=the amplitude of the fundamental harmonic of the optical signal,

V₂=the amplitude of the 2^(nd) order harmonic component of the opticalsignal, and

V_(dc)=the continuous incoherent component of the optical signal.

As before, the result of this operation, referenced as R, is a valuerepresenting the speed of angular displacement at which the gyroscopepivots around the supporting axle to which it is attached.

This operation can be performed as either an analog or a digitaloperation so the result can be used as an indication of the supportposition in relation to the pivot axle and in space in relation to areferential if three gyroscopes are used.

Until now, the description has concerned the structure of a gyroscopeaccording to the invention with a single axis of rotation, that is, asingle coil.

According to a variation shown schematically in FIG. 6, the gyroscopeaccording to the invention may comprise n axes, for example, three axes,one for each direction in space.

In this case, the gyroscope comprises as many reference coils B₁ throughB_(n) as there are corresponding axes 1 through n, coils B₁ throughB_(n) being connected to the unit consisting of light source 2 andinterference detector 6 by means of a star connector 18 with 2×2n accesspoints.

Star connector 18 with 2×2n access points between source 2, coils B₁through B_(n), and detector 6 is optional, since the emission surface ofthe source is large enough in relation to the diameter of the core ofthe fiber that the same amount of energy is injected in several fibersas if it were a single fiber. Thus, it is possible to treat a largenumber of circuits containing one coil as if they were so manyindividual circuits requiring only one X coupler per circuit, as shownin FIG. 7. Thus, all the structures of the complex, multiple starconnector are gained.

As in the case of the single-axis gyroscope, phase modulation means M₁through M_(n), respectively, each modulated by an excitation signal f₁through f_(n), respectively, are asymmetrically attached to one of theinputs of each coil B₁ through B_(n) (FIG. 8).

Next, a gyroscope with three axes—for example, one axis for eachdirection in space—will be examined.

Two different embodiments are envisioned.

According to the first of these, shown in FIG. 8, the inlets of thethree coils are supplied by light source 2 using a star coupler 19 with2×6 access points. Three sinusoidal oscillators denoted at OSC₁, OSC₂,and OSC₃ supply respective references f₁, f₂ and f₃ used to modulate theexcitation signals of three phase modulators M₁, M₂ and M₃asymmetrically attached to one of the inputs of each of the threegyroscope coils. Signals f₁, f₂ and f₃ delivered by oscillators OSC₁,OSC₂, and OSC₃ also serve as a reference for three synchronousdemodulators 20, 21 and 22 bypassing the output of interference detector6. As with the single axis gyroscope, the detection circuit is completedby a low pass filter 23 used for selection of the continuous incoherentcomponent V_(dc) of the optical signal. Finally, each synchronousdemodulator 20, 21 and 22 or double synchronous demodulator delivers oneor two output voltages V₁, V₂ and V₃ in the case of the two variationscorresponding to two quotients, above, and which furnish data about theSagnac phase for each of the three axes, that is, each of the threegyroscope coils.

According to the second possible embodiment shown in FIG. 9, a singlesinusoidal oscillator OSC furnishing a reference f is connected with aswitch 24 to each of the three phase modulators M₁, M₂ and M₃. Thus,modulation is accomplished in split time and the Sagnac data is obtainedsequentially for each coil with a single synchronous demodulator 25.

Certainly in the case of a three axis gyroscope, all the variations andoptions applicable to the single axis assembly remain valid andapplicable. The same is also true in the automatic control orcompensation module.

What is claimed is:
 1. A multimode optical fiber gyroscope comprising: aSAGNAC interferometer having an optical fiber coil comprising a lengthof multimode optical fiber having a first end and a second end; asymmetrical multimode X-coupler coupled to the first end and the secondend of the multimode optical fiber; a light source supplying lightthrough the a symmetrical multimode X-coupler into the first end and thesecond end of the multimode optical fiber coil, the multimode opticalfiber coil having an intermodal dispersion such that adjacent opticalpaths have a difference between co-propagating adjacent modes of anyorder of the supplied light that is greater than a coherence length ofthe light source, and a limited number of modes; an optical detector,coupled to the symmetrical multimode X-coupler, for receiving amultimode optical signal from the first and second ends of the opticalfiber via the symmetrical multimode X-coupler, the multimode opticalsignal being generated by a first and second counter rotating multimodeoptical signals emitted from the first and second ends of the multimodeoptical fiber; the optical detector receiving the multimode opticalsignal which is converted to an initial electrical signal, amplified andassigned a first voltage value V₁ proportional to the amplitude of thefirst order harmonic of the multimode optical signal, a second voltagevalue V₂ is assigned proportional to the second order harmonic of themultimode optical signal and a third voltage value V_(dc) is assignedproportional to the continuous incoherent component of the multimodeoptical signal; a processor for determining a result R based upon amathematical relationship between assigned voltage values V₁, V₂ andV_(dc) to compensate for fluctuations in the multimode optical signalcaused by intensity and wavelength instabilities in the light source,variability in the X-coupler, the multimode optical fiber and theoptical detector; and wherein the result R is determined for all themodes of the multimode optical signal and a statistical average resultis obtained.
 2. A multimode optical fiber gyroscope according to claim1, wherein the result R is determined by the processor based on themathematical relationship:$R = \frac{V_{1}}{\sqrt{{V_{2} \cdot V_{dc}}}}$

and a control loop provides a reference electrical signal for comparisonwith the initial electrical signal.
 3. The gyroscope according to claim1, wherein the multimode optical fiber has a step index profile.
 4. Thegyroscope according to claim 1, wherein the light source (2) is alight-emitting diode.
 5. The gyroscope according to claim 1, wherein thevoltages V₁ and V₂, proportional to the amplitudes of the 1^(st) and2^(nd) harmonics of the optical signal are detected by a synchronousdemodulator.
 6. The gyroscope according to claim 5, wherein an amplifierand a low pass filter are sequentially arranged at an output of theoptical detector (6) for isolating the voltage V_(dc) proportional tothe continuous incoherent component of the fiber optic signal.
 7. Thegyroscope according to claim 6, wherein a divider module establishes ascaling factor S based on the 1^(st) order harmonic after synchronousdemodulation and the voltage V_(dc) proportional to the continuousincoherent component of the fiber optic signal according to thealgebraic relationship: S=V ₁ /V _(dc).
 8. The gyroscope according toclaim 5, wherein the gyroscope further comprises a phase modulator (9)asymmetrically disposed on one of the first and second ends of the fiberoptic coil (1) and a sinusoidal oscillator (10) furnishes a referencevalue (ω_(i)) used to modulate any signal fluctuation of the phasemodulator (9).
 9. The gyroscope according to claim 8, wherein the phasemodulator (9) is a tube of piezoelectric material surrounded by a lengthof multimode optical fiber.
 10. The gyroscope according to claim 8,wherein the phase modulator (9) is a portion of multimode optical fiberwith an exterior coating of piezoelectric material.
 11. The gyroscopeaccording to claim 8, wherein the phase modulator (9) is a multimodefiber inserted in a flexible piezoelectric capillary.
 12. The gyroscopeaccording to claim 8, wherein the phase modulator (9) is a periodicmarking on a portion of fiber obtained by one of photogravure andchemical treatment.
 13. The gyroscope according to claim 8, wherein themodulation signal (ω_(i)) is applied as a reference signal to thesynchronous demodulator.
 14. The gyroscope according to claim 8, whereinthe light source (2) is modulated in amplitude at a modulation frequencyf which is lower than a modulation frequency of the phase modulator (9),and a rectangular double synchronous demodulator detects the continuousincoherent component V_(dc) of the optical signal.
 15. The gyroscopeaccording to claim 1, wherein the gyroscope comprises a single X coupler(5) which separates a wave emitted by the light source (2) into twosymmetrical counter-rotating modal distributions.
 16. The gyroscopeaccording to claim 15, wherein the X coupler is based upon interferencefilter properties.
 17. The gyroscope according to claim 15, wherein theX coupler is made from a diffractive plane, optical component.
 18. Thegyroscope according to claim 1, wherein the optical detector comprises adetection circuit having a synchronous demodulator (12) generating V₁and a low pass filter (15) which selects the continuous incoherentcomponent V_(dc) of the optical signal.
 19. The gyroscope according toclaim 18, wherein a divider (17) receives the V_(dc) signal from the lowpass filter (15) and the V₁ signal from the synchronous demodulator (12)to establish a scaling factor S from the algebraic relationship:S=V₁/V_(dc).
 20. The gyroscope according to claim 1, wherein thegyroscope functions at two different central wave lengths.
 21. Thegyroscope according to claim 1, wherein in order to stabilize a globalsignal of the gyroscope, a ratio L_(cf)/L is decreased, where L_(cf) isthe length of coherence in the optical fiber and L is the total lengthof the optical fiber.
 22. A multimode optical fiber gyroscopecomprising: a light source (2); an optical fiber ring guide comprised ofmultimode optical fiber; a coupler; a ring-shaped Sagnac interferometerattached in a non-reciprocal configuration with the light source (2)which supplies light to two opposed extremities of the optical fiberring guide via the coupler; an optical detector (6) for receiving twocounterpropagative modal distributions of the light once the lighttraverses the optical fiber ring guide in mutually inverse directions;the light is supplied through a single multimodal and modallysymmetrical coupler in both directions; the multimode optical fiber isan intermodal dispersion fiber such that an optical path differencebetween two nearby co-propagating modes of any order is much greaterthan a length of coherence of the light source (2) and a limited numberof possible propagation modes; the optical detector furnishes a voltagewith a value V₁ proportional to the amplitude of a first order harmonicto which is added a value V₂ proportional to the amplitude of the 2^(nd)order harmonic of the output signal of the optical detector (6) calledthe optical signal, and a value V_(dc) proportional to the incoherent DCcomponent of the optical signal obtained after amplification of theoptical signal; a processor for establishing a mathematical relationshipR between the values V₁, V₂, V_(dc), respectively, of the 0^(th), 1^(st)and 2^(nd) order harmonics to eliminate fluctuations in components ofthe gyroscope; wherein $R = \frac{V_{1}}{\sqrt{{V_{2} \cdot V_{dc}}}}$

and the modes and their combinations are averaged.
 23. The gyroscopeaccording to claim 22, wherein the coupler comprises a single Y coupler(7).
 24. The gyroscope according to claim 23, wherein the Y coupler isbased upon interference filter properties.
 25. The gyroscope accordingto claim 23, wherein the Y coupler is made from a diffractive plane,optical component.
 26. The gyroscope according to claim 22, wherein thegyroscope comprises a plurality of optical fiber ring guides (B₁, . . ., B_(n)) and the coupler is a star coupler (18) with 2×2n access points.27. The gyroscope according to claim 26, wherein the light source (2)and the detector (6) are each individually connected, through a singleindividual X coupler, to each of the plurality of optical fiber ringguides (B₁, . . . , B_(n)).
 28. The gyroscope according to claim 26,wherein the gyroscope comprises a single synchronous demodulator (25)and a single sinusoidal oscillator (OSC) connected to the plurality ofoptical fiber ring guides (B₁, B₂, B₃) by a switch (24).
 29. A multimodeoptical fiber gyroscope comprising: a SAGNAC interferometer having anoptical fiber coil comprising a length of multimode optical fiber havinga first end and a second end; a symmetrical multimode X-coupler coupledto the first end and the second end of the multimode optical fiber; alight source non-reciprocally supplying light through the symmetricalmultimode X-coupler into the first end and the second end of themultimode optical fiber, the multimode optical fiber having anintermodal dispersion such that adjacent optical paths have a differencebetween co-propagating adjacent modes of any order of the supplied lightthat is greater than a coherence length of the light source, and alimited number of modes; an optical detector, coupled to the symmetricalmultimode X-coupler, for receiving a multimode optical signal from thefirst and second ends of the optical fiber via the symmetrical multimodeX-coupler, the multimode optical signal being generated by a first andsecond counter rotating multimode optical signals emitted from the firstand second ends of the multimode optical fiber; the optical detectorreceiving the multimode optical signal which is converted to an initialelectrical signal, amplified and assigned a first voltage value V₁proportional to the amplitude of the first order harmonic of themultimode optical signal, a second voltage value V₂ is assignedproportional to the second order harmonic of the multimode opticalsignal and a third voltage value V_(dc) is assigned proportional to thecontinuous incoherent component of the multimode optical signal; aprocessor for determining a result R based upon a mathematicalrelationship between assigned voltage values V₁, V₂ and V_(dc) tocompensate for fluctuations in the multimode optical signal caused byinstability in the light source, variability in the X-coupler, themultimode optical fiber and the optical detector; wherein the result Ris determined for each mode of the multimode optical signal by theprocessor based on the mathematical relationship:$R = \frac{V_{1}}{\sqrt{{V_{2} \cdot V_{dc}}}}$

a statistical average result is obtained; and a control loop provides areference electrical signal for comparison with the initial electricalsignal.